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In 1931 Thornthwaite devised a complex and empirical classification, which is very close to Koppen's scheme. It also attempts to define climatic boundaries quantitatively and is based on plant associations.
However, Thornthwaite's classification is based on precipitation effectiveness and thermal efficiency (temperature efficiency). Under this classification climatic types were subdivided by the use of a term to denote the seasonal distribution of precipitation. The climatic types and their boundaries were defined empirically by observing the characteristics of natural vegetation, soil, and the drainage pattern.
Thornthwaite established the fact that not only the amount of precipitation, but the rate of evaporation as well is significant for the growth of natural vegetation. Thus, besides the precipitation amount and the evaporation rate, temperature was made a very important basis for Thornthwaite's climatic classification. An expression for precipitation efficiency was obtained by relating measurements of pan evaporation to temperature and precipitation.
For each month the ratio 11.5 (rt-10)10/9 where r=mean monthly rainfall (in inches) t=mean monthly temperature (in °F)
is calculated.
2. Main article
The sum of the 12 monthly ratios gives the precipitation effectiveness (also called precipitation efficiency) index. In other words, the effectiveness of precipitation is taken to be a function of precipitation and evaporation and is calculated by dividing the monthly precipitation by the monthly evaporation to get the P/E ratio (precipitation effectiveness ratio).
On the basis of P/E indices and boundary values for the major vegetation regions, five humidity provinces were defined. Main Climatic groups based on precipitation effectiveness
Humidity Province Vegetation P/E Index
A (Wet) Rain Forest 127
B (Humid) Forest 64-127
C (Sub humid) Grassland 32-63
D (Semiarid) Steppe 16-31
E (Arid) Desert 16
Thornthwaite introduced an index of thermal efficiency which is expressed by the positive departure of monthly mean temperatures from the freezing point The index is thus the annual sum of (t-32)/4 for each month. In other words, the sum of twelve monthly temperature-efficiency ratios (T/E) gives a T/E index
3. References
Again, the world was divided into 6 temperature provinces on the basis of T/E index. Main Climatic groups based on thermal efficiency
Temperature Province T/E index
A-Tropical 127
B-Mesothermal 64-127
C-Microthermal 32-63
D-Taiga 16-31
E-Tundra 1-15
F-Frost 0
T/E Index-sum of 12 monthly values of (T-32)14, where T is mean monthly temperature in °;F.
On the basis of the seasonal distribution of precipitation the humidity provinces were subdivided into the following
r-Rainfall adequate in all seasons
s-Rainfall deficient in summer
w-Rainfall deficient in winter
d-Rainfall deficient in all seasons.
When precipitation effectiveness, seasonal distribution of rainfall, and thermal efficiency are taken together, there would be in all 120 climatic types, at least on theoretical grounds. However, Thornthwaite has shown only 32 climatic types on the world map depicting his 1931 climatic classification.
